1. Field of the Invention
This invention relates to processes for preparing polymeric materials and polymeric materials so prepared and more particularly to processes for converting low molecular weight polyimides, with minimum elimination of by-products, to high molecular weight, thermally-stable polymers.
2. Prior Art
The synthesis in recent years of a number of thermally-stable polymers (e.g. polyimides) has supplied materials whose properties allow them to meet some critical end-use requirements. Their application to other uses is limited by a number of their specific properties, among which is intractability. This necessitates their use in dilute solutions in such exotic solvents as sulfuric acid, for example, to be spun into fibers. This excludes their use in laminations and in moldings. In addition, once isolated from the solvent, high temperatures are required to convert them to the fully condensed or cured final state. This curing is accompanied by the elimination of volatile by-products.
In contrast, polymers containing oxirane structures, such as epoxy-phenolics, can be cured at reasonably low temperatures with a minimum of by-products. However, the thermal stabilities of the oxirane polymers fall below those, for example, of the stable polybenzimidazoles. Thus, it is desirable to prepare oligomers which will propagate to higher molecular weights with minimum volatile elimination at relatively low temperatures and which will possess thermal stabilities reasonably higher than epoxy phenolics, preferably approaching the stabilities demonstrated by the polyimides and similar polyheterocyclics.